Much information can be found on the internet (some good, some bad) on Harp amplifiers and how to achive the best tone. As and amplifier technician and Blues Harp player I have carried out extensive research and I hope the following information will be of use to both players and technicians.
It is a common misconception that plugging your harp mic into any valve guitar amplifier will give you a good harp tone. Guitar pickups have a much lower output than hap style microphones leading the amplier to feedback and be too overdriven. Also guitar amplifiers are voiced specifically for the EQ spectrum of a guitar (very wide). Harps are very different demanding a more focussed tonal range limitied to the bass and mid frequencies. Most harp players will find when they plug in to a guitar amplifier that they have to increase both the bass and mid controls and roll the treble completely off.
Options for achieveing a good tone
Firstly your choice of amplifier is very important. try to choose and amplifier which learns towards being used with a harp. There are many 1 & 2 gain stage amplifiers which work well with harps which require little or no modifications to achieve optimum performance. The ideal amplifiers I would recommend are the early tweed Fenders, The Champ, Princeton as well as amplifiers by manufacturers such as Silvertone, Gibson, Magnatone, Supra and Harmony
Modifying a Guitar Amplifier for Harp use
Not all amplifiers are suitable for adaptation to be used with a harp. please feel free to get in touch with me if you are looking to purchase an amplifier and I will try to give my best honest advice on what can be done to improve the tonal response of each amplifier.
Rosewell Harp Amplifiers
After many years of both plying through and working on small valve amplifiers I have developed my own amplifier wich is specifically designed to be used eith a harp. This amplifier offered the focussed tonal response need by a harp amp without modification. for further details please check out the Harp Man 25 on the Custom Shop Page.